The download button opens the iTunes App Store, where you
may continue the download process. You must have iTunes installed with an
active iTunes account in order to download and install the software. This
download may not be available in some countries.
From chen xiaoyu:
Remember a pixel image and repaint it (demo). Tap to view original image again and tap to change the color of the pixel.Recreate the example image by painting squares in colors. Press to view the example image again and press to change color of your color gun. Shoot a square again to remove the color.